Safety relays are used for the purpose of safely breaking a circuit, for instance the power supply circuit of presses, machine tools, furnaces or medical appliances To this end, prior art installations employ independent contact sets connected in series, which contact sets are normally-open contacts of two independent monostable relays. In case one of the contact sets does not open, for instance due to contact welding, the circuit will still be broken by the other series-connected contact set of the second relay The safety may be increased by connecting more than two contact sets in series, although it is regularly assumed that the same error does not simultaneously occur at two contacts.
For recognizing a failure, each of the known safety relays is provided with a control contact set which is ganged to the load-driving contact set to indicate the position of the load-driving contact set irrespective of the position of the relay armature. The control contact sets of both relays are usually inserted in a control or evaluating circuit in such a manner that a renewed closure of the load circuit is prevented in case one of the load-driving contact sets has become welded. The condition of the control contact sets may be evaluated by means of a capacitor which, in the inoperative position of one relay, is charged via the control contact set operated as a normally-closed contact, the charge being required for switching the respective other relay to its operative position. In more sophisticated control circuits, there is an increasing tendency to employ microprocessors for evaluating the condition of the control contact sets.
Known safety relays usually include further contact sets which serve as holding contacts and as contacts in a signalling circuit for supervising the function of the system. All these contact sets (load-driving, control, holding and signalling contact sets) are regularly ganged and actuated in common by the relay armature.